1852: The sunspot number

<i>Sunspot drawings by Johann Hieronymus Schroeter (1745-1816),
an active solar observer between 1785 and 1795.
Schroeter's sunspot drawings were a primary source for Wolf's
reconstruction of activity cycle number 4 (1785--1798)</i>

As
<a href="images/schwabe.html"> Schwabe</a>'s
discovery of the sunspot cycle gained recognition,
the question immediately arose as to whether the cycle could
be traced farther in the past on the basis of extant sunspot
observations. In this endeavour the most active
researcher was without doubt the Swiss astronomer
<a href="images/wolf.html">Rudolf Wolf</a>
(1816-1893). Faced with the daunting task of
comparing sunspot observations carried out by many different
astronomers using various instruments and observing techniques,
Wolf defined the <b>relative sunspot number</b> (<i>r</i>)
as follows:

<p>
<center><i>r=k(f+10g)</i></center>

<p>where <i>g</i> is the number of sunspots groups visible on the
solar disk, <i>f</i> is the number of individual sunspots (including
those distinguishable within groups), and <i>k</i> is a correction
factor that varies from one observer to the next (with <i>k=1</i>
for Wolf's own observations, by definition). This definition
is still in used today, but <i>r</i> is now usually called
the Wolf (or Zurich) sunspot number. Wolf succeeded in
reliably reconstructing the variations in sunspot number as far
as the The 1755--1766 cycle, which has has since been known
conventionally as
"Cycle 1", with all subsequent cycles numbered consecutively thereafter;
at this writing (January 2000), we are in the rising phase of cycle 23.